During mobile communication, a mobile station performs communications with network apparatuses, the network apparatuses include a base station apparatus and a radio network controller. The network apparatuses may include a core network. The mobile station detects multiple cells that can be targets for communications or candidates for communications, and measures radio quality in the cells. When a measured value of the radio quality of a communication candidate cell satisfies a predetermined condition, a communication target cell is switched. For example, when it is determined that a newly detected cell will provide relatively better radio quality than that of a cell that has already been performing communications, the communication target cell is switched to the newly detected cell.
A high-speed downlink packet access (HSDPA) scheme realizes high-speed down link communication for the wideband code division multiple access (W-CDMA) communication scheme. In this case, an A-DPCH (Associated Dedicated Physical CHannel) simultaneously performs communications with up to three cells. However, an HS-PDSCH (High Speed-Physical Downlink Shared CHannel) sets only one cell that has the best radio quality as a communication target. The cell which has been selected as the cell having the best quality is called a “Serving Cell.” Since the A-DPCH is the communication based on a dedicated channel, a soft handover is performed. While maintaining the communication with the serving cell, only the switching target cell is switched depending on necessity. On the other hand, since the HS-PDSCH is the communication based on a down link shared or common channel (Shared channel), there is only one cell that can be a communication target. Therefore, when a cell is switched due to, for example, movement of a user, a handover is performed, and thereby a short interruption occurs temporarily. A conventional handover is described in Patent Document 1, for example.
During communication based on the HSDPA scheme, the mobile station measures quality (such as SIR) of a CPICH, which is a pilot channel in the serving cell. The mobile station derives a channel quality indicator (CQI: Channel Quality Indicator) based on the measurement result. The derived CQI is reported from the mobile station to the network apparatus. As a result, the network apparatus can recognize a reception environment (radio quality) of the mobile station, and the mobile station can adaptively switch a modulation scheme and a transmission data amount for down link data, depending on the environment. It is called adaptive modulation and channel coding to adaptively switch the data modulation scheme and the amount of data (or the channel coding rate). Relationships of the down link data modulation scheme and a transport block size (Transport Block size) with respect to individual values of the CQI are defined in a 3GPP specification (cf. Non-Patent Document 2).